Helicobacter pylori is a Gram negative, microaerophilic gastric pathogen that infects nearly 50% of the human population. It is the sole pathogen able to colonize the stomach, and has been closely linked to duodenal and gastric ulcers and adenocarcinomas. The bacterium has been listed as a Group 1 carcinogen by the World Health Organization. The CagA cytotoxin of H. pylori is translocated into host cells by a Type IV Secretion System, interacts with many cellular proteins, increases virulence, and is associated with an increased risk of cancer. CagA has been shown to manipulate many host cellular functions, including cytoskeletal function, cell- to-cell adhesion, and intracellular signl transduction. However, how CagA functions has remained mostly a mystery for nearly two decades. Preliminary data from our laboratory is changing this situation. We have determined high-resolution crystal structures of CagA bound to host targets, have identified stable, soluble subdomains of this large cytotoxin, and have identified novel host factors that interact with these subdomains. This application proposes to perform biochemical, structural, and infection biological studies of these identified subdomains and their interactions with host factors. The specific aims are (1) to identify, biochemically and structurally characterize the CagA functional subdomains, and (2) to identify, biochemically and structurally characterize the CagA-host factor interactions. Expected outcomes include obtaining detailed crystal structures of CagA host factor interactions, biochemical characterization of the nature of these interactions, and the role of the interactions in modulating host cell biology. Such data will greatly enhance our understanding of virulence, as well as Helicobacter driven carcinogenesis.